Introduction: Koi Fish Pond Bio-filter

This is my first Instructable, I'm not an expert and this pond filter experiment has been pulled together from other Instructables and internet research. There's no guaranteeing this will work for everyone, but it's working for me.

Bit of background, I have a 14" by 5" kidney shaped and stepped pond, roughly 10,000 litres. I've got a 13,000 litre per hour submersible pump that was running through a 10,000 litre UV & bio-filter (hozelock). I get a lot of sunlight on my pond and the battle with the blanket weed and other gunk was one I was losing. I was constantly having to scrub my foam filters to keep the water even anywhere near clear. So i decided to upgrade my filtration, but the expense of a bigger filter was a factor.

So before I splashed out the cash for a much bigger filter, I thought I'd have a stab at making one myself.

Step 1: What You'll Need.

Most of what you need can be bought from any hardware store (in the UK Screwfix is cheaper than B&Q) and any good pond/fisheries garden center. failing that eBay.

I've built this filter out of standard plumbing supplies that I had lying around in my workshop from previous DIY projects, the only specialist part is the barrel I've used, but any large heavy duty plastic container should work (wheelie bin, water butt etc)



40mm hole cutting drill bit (I used a 41mm)

10mm drill bit

pipe spanner

sharp knife


For the Main filter you'll need:

1 x large heavy duty plastic barrel

1 x large plastic bucket or bin that will just fit inside the barrel

40mm waste pipe (I used a couple of metres of this mainly to reach my pond)

4 x 40mm waste tank connector

2 x 40mm t-piece push fit connectors

4 x 40mm 90 degree elbow push fit connectors

2 x bags of medium sized stones (I used small cobble stones under 1 inch in size)

2 x bags of small sized stones (I used pea gravel)

Lots of sponges (I used around 100)

Large piece of foam (big enough to fit tightly in the barrel)

1 x Silicone sealant (and a sealant gun)

I also ended up using some firebglass repair kit on the main seals, as I was a bit off center when I drilled one of the holes.

Optional cleaning/flushing system

4 x 21mm 90 degree elbows (solvent weld type)

3 x 21mm t-piece (solvent weld type)

2 metres of 21mm pipe (solvent weld type) you might need more or less depending on the size/height of the main barrel.

1 x pipe adhesive

Step 2: Cut, Drill & Fit Your Barrel

First off I needed to remove the top of my barrel as it was bonded onto the body.

I took an angle grinder and sliced off the top, inside of the rim and tidied up the rough edges.

Using the 40mm hold cutting drill bit, you're going to want to drill at least three holes.

Make sure if you're drilling into a round barrel that you're nice and square when you drill, otherwise you'll get an elliptical hole..

The first one needs to be low down, approx 6 inches up from the bottom of the barrel to allow for the internal pipe work we'll build in a minute and also give you some room for the dirt to settle.

The second and third need to be near the top, one a couple of inches from the top of the barrel, this will be our overflow in case of emergencies and the other about 6 inches from the top of the barrel, this will be the return to the pond outlet.

Once drilled fit the 40mm fittings ensuring that they're bolted on good and secure and that they're water tight, if in doubt go nuts with the silicone sealant you can't be too generous here. I also used fiberglass as I got one of my holes slightly off square and the fitting didn't quite cover the gap.

Leave this overnight someplace warm and dry to ensure everything has dried and set properly.

Step 3: Build the Internal Vortex Pipes

To make the water swirl around in the bottom of the barrel I added the vortex, this way you should get some rotation of the water as is enters the system which will aid in separating the bigger bits of dirt out via centrifugal force. i don't know how effective this really is, it all depends on the power of the pump you're using but it looked cool so I made one.

Cutting some short sections (10cm to 20cm) you can attach the push fit 40mm elbows to the T-piece to build the vortex. you'll need to adjust the size to fit your own barrel.

Cut a length of 40mm pipe so that the vortex sits in the middle of the barrel.

I also added a support piece, cut from 40mm pipe and glued onto the underside of the vortex just to hold it up off the bottom of the barrel and to not stress the hole/fitting.

It's worth a test at this point to make sure the pipes hold together and the water swirls around, I used a hosepipe just to prove it all worked.

Step 4: Optional Cleaner/flusher

This is optional, but I added it so I don't have to dig out all the filter stones whenever I want to clean the filter.

The idea is that this will allow you to blow water, or air, through these pipes to flush the filter out, forcing dirt to the top of the filter where you can drain it off.

Using the 21mm pipe cut 5 equal lengths that will make a square that will fit inside the barrel. In mine I made a 40cmx40cm frame to fit inside the 60cm barrel

You'll need to cut three of the lengths in half to insert the T-pieces, when cutting you'll also have to account for the size of the T-pieces and cut out that little bit extra (approx 2cms)

Build the frame (as shown) using plenty of the adhesive.

Cut a length of 21mm pipe that is a good 6 inches taller than the main barrel and glue this in place and leave to dry somewhere for a few hours.

Once dry you can drill holes along the frame, I roughly measured so they are equally spaced out.

Test place it in the barrel to ensure it fits around the vortex pipework.

Step 5: Vortex Shield & Stone Holder

So that when you add the filtration media stones you don't crush, snap or block the lovely vortex we made earlier you'll need to protect it with a bucket.

I had an old large plant pot from a bamboo lying around, but any large bucket or bin could be cut to suit. It will need to be a nice fit in the bottom of the barrel and also still leave a couple of inches clearance for the vortex pipework.

You need to drill plenty of holes in the bucket, of varying sizes and position, don't go mad and put too many in as this will compromise the buckets strength, remember you've got 4 bags of rocks to drop in yet.

You will also need to cut a slot for the vortex pipework to fit though.

In mine I've cut a stepped slot, a little like a bayonet light bulb fitting, so when I twist the bucket it will lock into place and also support the vortex pipe

Try a test fitting, I had to modify mine a bit to get it to fit.

If you've made the flusher I'd also put this in and check there's a hold in the right place and big enough to get the bucket over it and still fit around the vortex pipe.

Step 6: Adding the Filter Media

Ok, so everything should fit together nicely now, the vortex is installed, the flusher (if you've made one) is in and the bucket is all slotted into place nicely. Double and triple check everything is in place as you don't want to have to dig out all the stones if you've forgotten something..

So it's time to add the filter media.

I used normal gravel and stone from a local garden center but there are more pond friendly products available should you choose to buy them (larva rock etc) but as I'm on a budget I've gone for the cheap option. (I may upgrade later)

Start slowly adding the larger set of stones and rocks until the bucket is covered, then add the smaller stones until the barrel is about half filled with stones.

Next you can put in the sponges, I've used lots of cheap scourer pad sponges from the local £1 shops, but you can splash out and use proper Japanese matting foams here if you wanted.

Finally to cap it all off and stop the smaller sponges just floating away I cut a large piece of foam that I had lying around down to the size of the barrel using the lid as a template, cut a slot for the flusher pipe and wedged this in to hold everything down into the water.

Test fill the barrel, what I found was the foam plug I made didn't hold down all the small sponges so I used some rocks to weight it down.

Step 7: Connecting Into Your Pond

So now it's time to hook this up to your pond and pump.

On the lowest connection you made on the outside of the barrel you're going to want to connect your pump. I run a 1 inch flexi pipe from my submersible pump up to the filters, so I had to adapt the 40mm input pipe to accommodate a 1 inch flexi pipe.

I did this using a pond pump adapter, silicone sealant and fiberglass bonded together to allow my 1 inch pipe to be slotted onto the bottom of the barrel.

The top outlet I've plumbed straight back into my pond using 40mm pipework, this is the overflow

The lower of the top two outlets I've plumbed into the inlet of my existing 10,000 litre UV filter, this way I get 50% of the filtered water also get a UV blast and further filtration

Turn on your pump and test it out.

Step 8: Lessons Learnt

Once I'd connected up my new filter and turned on the pump I soon realised that a 13,000 litre per hour pump was too much for the barrel to handle and it swamped the two outlets I had going to the second filter and overflow back into the pond. The barrel didn't overflow but I was using both my outlets and had no backup.

So I added a fourth hole into the barrel midway between the top two outlets. So now I have two overflows one above the other. I've plumbed them in together so I only need one outlet pipe back into the pond but I've still got a backup should the barrel start to overflow and this is working nicely.

thanks for reading my first Instructable, good luck :)